Type Ia Supernova Models Research Articles

A Three‐Dimensional Deflagration Model for Type Ia Supernovae Compared with Observations

A simulation of the thermonuclear explosion of a Chandrasekhar-mass C+O white dwarf, the most popular scenario of a Type Ia supernova (SN Ia), is presented. The underlying modeling is pursued in a self-consistent way, treating the combustion wave as a turbulent deflagration using well tested methods developed for laboratory combustion and based on the concept of large-eddy (LESs). Such consistency requires to capture the onset of the turbulent cascade on resolved scales. This is achieved by computing the dynamical evolution on a 10243 moving grid, which resulted in the best-resolved three-dimensional SN Ia simulation carried out thus far, reaching the limits of what can be done on present supercomputers. Consequently, the model has no free parameters other than the initial conditions at the onset of the explosion, and therefore it has considerable predictive power. Our main objective is to determine to which extent such a simulation can account for the observations of normal SNe Ia. Guided by previous simulations with less resolution and a less sophisticated flame model, initial conditions were chosen that yield a reasonably strong explosion and a sufficient amount of radioactive nickel for a bright display. We show that observables are indeed matched to a reasonable degree. In particular, good agreement is found with the light curves of normal SNe Ia. Moreover, the model reproduces the general features of the abundance stratification as inferred from the analysis of spectra. This indicates that it captures the main features of the explosion mechanism of SNe Ia. However, we also show that even a seemingly best-choice pure deflagration model has shortcomings that indicate the need for a different mode of nuclear burning at late times, perhaps the transition to a detonation at low density.

Constraining Deflagration Models of Type Ia Supernovae through Intermediate‐Mass Elements

The physical structure of a nuclear flame is a basic ingredient of the theory of Type Ia supernovae (SNIa). Assuming an exponential density reduction with several characteristic times we have followed the evolution of a planar nuclear flame in an expanding background from an initial density 6.6 10^7 g/cm3 down to 2 10^6 g/cm3. The total amount of synthesized intermediate-mass elements (IME), from silicon to calcium, was monitored during the calculation. We have made use of the computed mass fractions, X_IME, of these elements to give an estimation of the total amount of IME synthesized during the deflagration of a massive white dwarf. Using X_IME and adopting the usual hypothesis that turbulence decouples the effective burning velocity from the laminar flame speed, so that the relevant flame speed is actually the turbulent speed on the integral length-scale, we have built a simple geometrical approach to model the region where IME are thought to be produced. It turns out that a healthy production of IME involves the combination of not too short expansion times, t_c > 0.2 s, and high turbulent intensities. According to our results it could be difficult to produce much more than 0.2 solar masses of intermediate-mass elements within the deflagrative paradigma. The calculations also suggest that the mass of IME scales with the mass of Fe-peak elements, making it difficult to conciliate energetic explosions with low ejected nickel masses, as in the well observed SN1991bg or in SN1998de. Thus a large production of Si-peak elements, especially in combination with a low or a moderate production of iron, could be better addressed by either the delayed detonation route in standard Chandrasekhar-mass models or, perhaps, by the off-center helium detonation in the sub Chandrasekhar-mass scenario.

Light curves for off-centre ignition models of Type Ia supernovae

Motivated by recent models involving off-centre ignition of Type Ia supernova explosions, we undertake three-dimensional time-dependent radiation transport simulations to investigate the range of bolometric light-curve properties that could be observed from supernovae in which there is a lop-sided distribution of the products from nuclear burning. We consider both a grid of artificial toy models which illustrate the conceivable range of effects and a recent three-dimensional hydrodynamical explosion model. We find that observationally significant viewing angle effects are likely to arise in such supernovae and that these may have important ramifications for the interpretation of the observed diversity of Type Ia supernova and the systematic uncertainties which relate to their use as standard candles in contemporary cosmology.

Detonating Failed Deflagration Model of Thermonuclear Supernovae. I. Explosion Dynamics

We present a detonating failed deflagration model of Type Ia supernovae. In this model, the thermonuclear explosion of a massive white dwarf follows an off-center deflagration. We conduct a survey of asymmetric ignition configurations initiated at various distances from the stellar center. In all cases studied, we find that only a small amount of stellar fuel is consumed during deflagration phase, no explosion is obtained, and the released energy is mostly wasted on expanding the progenitor. Products of the failed deflagration quickly reach the stellar surface, polluting and strongly disturbing it. These disturbances eventually evolve into small and isolated shock-dominated regions which are rich in fuel. We consider these regions as seeds capable of forming self-sustained detonations that, ultimately, result in the thermonuclear supernova explosion. Preliminary nucleosynthesis results indicate the model supernova ejecta are typically composed of about 0.1-0.25 Msun of silicon group elements, 0.9-1.2 Msun of iron group elements, and are essentially carbon-free. The ejecta have a composite morphology, are chemically stratified, and display a modest amount of intrinsic asymmetry. The innermost layers are slightly egg-shaped with the axis ratio ~1.2-1.3 and dominated by the products of silicon burning. This central region is surrounded by a shell of silicon-group elements. The outermost layers of ejecta are highly inhomogeneous and contain products of incomplete oxygen burning with only small admixture of unburned stellar material. The explosion energies are ~1.3-1.5 10^51 erg.

Constraining supernova models using the hot gas in clusters of galaxies

The hot Intra-Cluster Medium (ICM) in clusters of galaxies is a very large repository of metals produced by supernovae. We aim to accurately measure the abundances in the ICM of many clusters and compare these data with metal yields produced by supernovae. Using the data archive of the XMM-Newton X-ray observatory, we compile a sample of 22 clusters. We fit spectra extracted from the core regions and determine the abundances of silicon, sulfur, argon, alcium, iron, and nickel. The abundances from the spectral fits are subsequently fitted to supernova yields determined from several supernova type Ia and core-collapse supernova models. We find that the argon and calcium abundances cannot be fitted with currently favoured supernova type Ia models. We obtain a major improvement of the fit, when we use an empirically modified delayed-detonation model that is calibrated on the Tycho supernova remnant. The two modified parameters are the density where the sound wave in the supernova turns into a shock and the ratio of the specific internal energies of ions and electrons at the shock. Our fits also suggest that the core-collapse supernovae that contributed to the enrichment of the ICM had progenitors which were already enriched. The Ar/Ca ratio in clusters is a good touchstone for determining the quality of type Ia models. The core-collapse contribution, which is about 50% and not strongly dependent on the IMF or progenitor metallicity, does not have a significant impact on the Ar/Ca ratio. The number ratio between supernova type Ia and core-collapse supernovae suggests that binary systems in the appropriate mass range are very efficient (~ 5-16%) in eventually forming supernova type Ia explosions.

Spectropolarimetric Diagnostics of Thermonuclear Supernova Explosions

Even at extragalactic distances, the shape of supernova ejecta can be effectively diagnosed by spectropolarimetry. We present results for 17 type Ia supernovae that allow a statistical study of the correlation among the geometric structures and other observable parameters of type Ia supernovae. These observations suggest that type Ia supernova ejecta typically consist of a smooth, central, iron-rich core and an outer layer with chemical asymmetries. The degree of this peripheral asphericity is correlated with the light-curve decline rate of type Ia supernovae. These results lend strong support to delayed-detonation models of type Ia supernovae.

SN 1006 at optical and UV wavelengths

AbstractOptical and UV emission from SN 1006 is observed on the periphery of the SNR, particularly in the NW, and arises as material from interstellar gas is ionized behind the shock front. The shapes of the emission lines have been used to infer a shock velocity of about 2900 km s−1 and to show that the electron and ion temperatures are close to that expected from standard shock theory. The combination of the shock velocity and the proper motion of the filaments in the NE accurately locates SN1006 at a distance of 2.2 kpc, some 550 pc above the Galactic plane. At UV wavelengths, ejecta from SN 1006 have also been observed as broad absorption lines from Si, Ca and Fe in spectra of the Schweitzer-Middleditch star and two quasars. The observations appear to limit the mass of Fe in SN 1006 to less than 0.16 M⊙, much less than expected from models of Type Ia supernovae. In this brief review of the UV and optical properties of SN 1006, I will summarize how these observations yield a fairly consistent description of SN 1006, and suggest what further observations might be undertaken to extend our understanding of SN 1006 in its second millennium.

C+O detonations in thermonuclear supernovae: interaction with previously burned material

In the context of explosion models for Type Ia Supernovae, we present one- and two-dimensional simulations of fully resolved detonation fronts in degenerate C+O White Dwarf matter including clumps of previously burned material. The ability of detonations to survive the passage through sheets of nuclear ashes is tested as a function of the width and composition of the ash region. We show that detonation fronts are quenched by microscopically thin obstacles with little sensitivity to the exact ash composition. Front-tracking models for detonations in macroscopic explosion simulations need to include this effect in order to predict the amount of unburned material in delayed detonation scenarios.

The ignition of thermonuclear flames in type Ia supernovae

In the framework of the Chandrasekar-mass deflagration model for Type Ia supernovae, a persisting free parameter is the initial location of the flame front, which is linked to the ignition process in the progenitor white dwarf. Previous analytical models indicate that the thermal runaway is driven by temperature perturbations (bubbles'') developing in the white dwarf's convective core. In this work, numerical 2D simulations of buoyant bubbles in the white dwarf interior are presented and discussed, in order to probe the bubble features at ignition (diameters, temperatures and evolutionary time scales). The results support the multi-point scenario as the most favored ignition model for the SN Ia explosion.

Multi-spot ignition in type Ia supernova models

We present a systematic survey of the capabilities of type Ia supernova explosion models starting from a number of flame seeds distributed around the center of the white dwarf star. To this end we greatly improved the resolution of the numerical simulations in the initial stages. This novel numerical approach facilitates a detailed study of multi-spot ignition scenarios with up to hundreds of ignition sparks. Two-dimensional simulations are shown to be inappropriate to study the effects of initial flame configurations. Based on a set of three-dimensional models, we conclude that multi-spot ignition scenarios may improve type Ia supernova models towards better agreement with observations. The achievable effect reaches a maximum at a limited number of flame ignition kernels as shown by the numerical models and corroborated by a simple dimensional analysis.

On the Sensitivity of Deflagrations in a Chandrasekhar Mass White Dwarf to Initial Conditions

We analyze the sensitivity of the flame propagation in a Chandrasekhar mass white dwarf (WD) to initial conditions during the subsonic burning phase (deflagration) using two-dimensional simulations of the full WD. Results are presented for a wide variety of initial flame distributions, including central and off-center, single-point and multipoint, and simultaneous and nonsimultaneous ignitions. We also examine the effects of the convective velocity field that should exist at the core before thermonuclear runaway. Our main conclusion suggests that the amounts of burning products and their distributions through the deflagration phase are extremely sensitive to initial conditions, much more sensitive than those presented in previous studies. In particular, we find that more complex configurations, such as even slightly off-center ignitions, nonsimultaneous multipoint ignitions, and velocity fields, tend to favor solutions in which individual plumes rise faster than the bulk of a typical Rayleigh-Taylor-driven, unstable burning front. The difference from previous calculations for an octant of a WD may be understood as a consequence of the suppression of the l = 1 and 2 modes. Our results are consistent with full-star three-dimensional calculations by the Chicago group. Moreover, the total amount of nuclear burning during the phase of subsonic burning depends sensitively on the initial conditions and may cause the WD to become unbound or to pulsate. The latter case opens (again) a new parameter space for the Chandrasekhar WD problem. We discuss the implications of the results on current models for Type Ia supernovae and limitations imposed by the two-dimensional nature of our study and suggest directions for further study.

The p-Process Nucleosynthesis in the Carbon Deflagration Model for Type Ia Supernovae

We calculate nucleosynthesis of proton-rich isotopes in the Carbon deflagration model for Type Ia supernovae (SNe Ia). We investigate the influence of variations in the C/O ratios and 22 Ne abundance left in accreting CO white dwarf on the p -process nucleosynthesis and the sensitivity of the seed abundance distribution. We find that about 55% of the p -nuclides are produced at the levels that can explain the solar abundances in all adopted cases of initial compositions. Mo and Ru isotopes severely deficient in Type II supernovae (SNe II) are enhanced by a factor of ∼10, but unavoidable underproduction still exists. We also compare the yields of 56 Fe and p -nuclei in SNe Ia and SNe II, and obtain a result that C-deflagration supernovae can contribute to the galactic evolution (and the solar system) of the p -nuclei if the s -process occurs in accreting CO white dwarfs at the similar efficiency as in AGB stars.

Probing Multiple Sight Lines through the SN 1006 Remnant by Ultraviolet Absorption Spectroscopy

Absorption-line spectroscopy is an effective probe for cold ejecta within a supernova remnant (SNR), provided that suitable background UV sources can be identified. For the SN 1006 remnant we have identified four such sources, in addition to the much-studied Schweitzer-Middleditch (SM) star. We have used STIS on the Hubble Space Telescope to obtain UV spectra of all four sources, to study core samples of the SN 1006 interior. The line of sight closest to the center of the SNR shell, passing only 20 away, is to a V = 19.5 QSO at z = 1.026. Its spectrum shows broad Fe II absorption lines, asymmetric with red wings broader than blue. The similarity of these profiles to those seen in the SM star, which is 28 from the center in the opposite direction, confirms the existence of a bulge on the far side of SN 1006. The Fe II equivalent widths in the QSO spectrum are ~50% greater than in the SM star, suggesting that somewhat more iron may be present within SN 1006 than studies of the SM star alone have indicated, but this is still far short of what most Type Ia supernova models require. The absorption spectrum against a brighter z = 0.337 QSO seen at 57% of the shell radius shows broad silicon absorption lines but no iron other than narrow, probably interstellar lines. The cold iron expanding in this direction must be confined within v 5200 km s-1, also consistent with a high-velocity bulge on the far side only. The broad silicon lines indicate that the silicon layer has expanded beyond this point, and that it has probably been heated by a reverse shock—conclusions consistent with the clumpy X-ray structure and anomalous abundances observed from Chandra in this region. Finally, the spectra of two ~A0 V stars near the southern shell rim show no broad or unusually strong absorption lines, suggesting that the low-ionization ejecta are confined within 83% of the shell radius, at least at the azimuths of these background sources.

Thermal X‐Ray Emission from Shocked Ejecta in Type Ia Supernova Remnants. II. Parameters Affecting the Spectrum

The supernova remnants left behind by Type Ia supernovae provide an excellent opportunity for the study of these enigmatic objects. In a previous work, we showed that it is possible to use the X-ray spectra of young Type Ia supernova remnants to explore the physics of Type Ia supernovae and identify the relevant mechanism underlying these explosions. Our simulation technique is based on hydrodynamic and nonequilibrium ionization calculations of the interaction of a grid of Type Ia explosion models with the surrounding ambient medium, coupled to an X-ray spectral code. In this work we explore the influence of two key parameters on the shape of the X-ray spectrum of the ejecta: the density of the ambient medium around the supernova progenitor and the efficiency of collisionless electron heating at the reverse shock. We also discuss the performance of recent 3D simulations of Type Ia SN explosions in the context of the X-ray spectra of young SNRs. We find a better agreement with the observations for Type Ia supernova models with stratified ejecta than for 3D deflagration models with well mixed ejecta. We conclude that our grid of Type Ia supernova remnant models can improve our understanding of these objects and their relationship to the supernovae that originated them.

Lower limits on the Hubble constant from models of type Ia supernovae

By coupling observations of type Ia supernovae with results obtained from the best available numerical mod- els we constrain the Hubble constant, independently of any external calibrators. We find an absolute lower limit of H0 > 50 km s −1 Mpc −1 . In addition, we construct a Hubble diagram with UVOIR light curves of 12 type Ia supernovae located in the Hubble flow, and when adopting the most likely values (obtained from 1-D and 3-D deflagration simulations) of the amount of 56 Ni produced in a typical event, we find values of H0 ≥ 66 ± 8a nd≥78 ± 9k m s −1 Mpc −1 , respectively. Our result may be difficult to reconcile with recent discussions in the literature as it seems that an Einstein-de Sitter universe requires H0 � 46 km s −1 Mpc −1 in order to fit the temperature power spectrum of the cosmic microwave background and maintain the age constraints of the oldest stars.

Type Ia Supernova models arising from different distributions of igniting points

In this paper we address the theory of type Ia supernovae from the moment of carbon runaway up to several hours after the explosion. We have concentrated on the boiling-pot model: a deflagration characterized by the (nearly-) simultaneous ignition of a number of bubbles that pervade the core of the white dwarf. Thermal fluctuations larger than $\ga$1% of the background temperature (~$7\times10^8$ K) on lengthscales of $\le $1 m could be the seeds of the bubbles. Variations of the homogeneity of the temperature perturbations can lead to two alternative configurations at carbon runaway: if the thermal gradient is small, all the bubbles grow to a common characteristic size related to the value of the thermal gradient, but if the thermal gradient is large enough, the size spectrum of the bubbles extends over several orders of magnitude. The explosion phase has been studied with the aid of a smoothed particle hydrodynamics code suited to simulate thermonuclear supernovae. In spite of important procedural differences and different physical assumptions, our results converge with the most recent calculations of three-dimensional deflagrations in white dwarfs carried out in supernova studies by different groups. For large initial numbers of bubbles ($\ga$3–4 per octant), the explosion produces ~$0.45 \;M_{\odot}$ of 56 Ni, and the kinetic energy of the ejecta is ~$0.45\times10^{51}$ ergs. However, all three-dimensional deflagration models share three main drawbacks: 1) the scarce synthesis of intermediate-mass elements; 2) the loss of chemical stratification of the ejecta due to mixing by Rayleigh-Taylor instabilities during the first second of the explosion; and 3) the presence of big clumps of 56 Ni at the photosphere at the time of maximum brightness. On the other hand, if the initial number of igniting bubbles is small enough, the explosion fails, the white dwarf oscillates, and a new opportunity comes for a detonation to ignite and process the infalling matter after the first pulsation.

The distributed burning regime in type Ia supernova models

The deflagration mode of flame propagation in type Ia supernova (SN Ia) models requires a correct description of the interaction of the flame with turbulent motions. It is well-known that turbulent combustion proceeds in different regimes. For most parts of the deflagration in SNe Ia the flamelet regime applies. This has been modeled in previous multi-dimensional simulations. However, at late stages of the explosion, the flame will propagate in the so-called distributed regime. We investigate the effects of this regime on SN Ia models in a first and simplified approach and show that the trend of effects seems capable of curing some problems of current pure deflagration models.

XMM-Newtonobservations of the supernova remnant RX J0852.0-4622/GRO J0852-4642

RX J0852.0-4622 is a supernova remnant discovered in the ROSAT all-sky survey. Spatially coincident 1.157 MeV γ-ray line emission was detected with the COMPTEL instrument on-board CGRO. The analysis combining the X-ray and γ-ray data suggests that RX J0852.0-4622 is a close-by and young supernova remnant. Follow-up observations with ASCA show that the two brightest sections of the limb have non-thermal spectra, which make an independent assessment of the age and distance using the Sedov equations for the evolution of the remnant almost impossible. We have observed three rim sections of RX J0852.0-4622 with XMM-Newton and confirm the power-law type spectra measured with ASCA. We also confirm the presence of an emission-line like feature at keV, which we suggest to be emission from Ti and Sc excited by atom/ion or ion/ion high velocity collisions. The high velocity is in agreement with the width of the 1.157 MeV γ-ray line. The X-ray line flux expected from such an interaction is consistent with the 1.157 MeV γ-ray line flux measured by COMPTEL. This consistency of the X-ray line flux and the γ-ray line flux lends further support to the existence and amounts of Ti in RX J0852.0-4622 claimed by Iyudin et al. ([CITE], Nature, 396, 142) and to the suggestion that RX J0852.0-4622 is young and nearby (Aschenbach et al. [CITE], A&A, 350, 997). Iyudin et al. ([CITE]) quote a very large broadening of the 1.157 MeV γ-ray line which would indicate a large velocity of the emitting matter of about 15 000 km s-1. Such high ejecta velocity for Ti is found only in explosion models of sub-Chandrasekhar type Ia supernovae (Woosley & Weaver [CITE], ApJ, 423, 371; Livne & Arnett [CITE], ApJ, 452, 62). In this case no compact remnant is expected. The obvious, remaining questions are what the nature and the origin of the central compact source CXOU J085201.4-461753 are and why the absorption column density apparently associated with RX J0852.0-4622 is much greater than the typical column for the Vela SNR.

Three‐dimensional Delayed‐Detonation Model of Type Ia Supernovae

We study a Type Ia supernova explosion using large-scale three-dimensional numerical simulations based on reactive fluid dynamics with a simplified mechanism for nuclear reactions and energy release. The initial deflagration stage of the explosion involves a subsonic turbulent thermonuclear flame propagating in the gravitational field of an expanding white dwarf. The deflagration produces an inhomogeneous mixture of unburned carbon and oxygen with intermediate-mass and iron-group elements in central parts of the star. During the subsequent detonation stage, a supersonic detonation wave propagates through the material unburned by the deflagration. The total energy released in this delayed-detonation process, (1.3-1.6) × 1051 ergs, is consistent with a typical range of kinetic energies obtained from observations. In contrast to the deflagration model, which releases only about 0.6 × 1051 ergs, the delayed-detonation model does not leave carbon, oxygen, and intermediate-mass elements in central parts of a white dwarf. This removes the key disagreement between three-dimensional simulations and observations, and makes a delayed detonation the mostly likely mechanism for Type Ia supernova explosions.

Thermal Timescale Mass Transfer and the Evolution of White Dwarf Binaries

The evolution of binaries consisting of evolved main sequence stars (1 < M_d/Msun < 3.5) with white dwarf companions (0.7 < M_wd/Msun < 1.2) is investigated through the thermal mass transfer phase. Taking into account the stabilizing effect of a strong, optically thick wind from the accreting white dwarf surface, we have explored the formation of several evolutionary groups of systems for progenitors with initial orbital periods of 1 and 2 days. The numerical results show that CO white dwarfs can accrete sufficient mass to evolve to a Type Ia supernova and ONeMg white dwarfs can be built up to undergo accretion induced collapse for donors more massive than about 2 Msun. For donors less massive than ~2 Msun the system can evolve to form a He and CO or ONeMg white dwarf pair. In addition, sufficient helium can be accumulated (~0.1 Msun) in systems characterized by 1.6 < M_d/Msun < 1.9 and 0.8 < M_wd/Msun < 1 such that sub Chandrasekhar mass models for Type Ia supernovae, involving off center helium ignition, are possible for progenitor systems evolving via the Case A mass transfer phase. For systems characterized by mass ratios > 3 the system likely merges as a result of the occurrence of a delayed dynamical mass transfer instability. A semi-analytical model is developed to delineate these phases which can be easily incorporated in population synthesis studies of these systems.